4 Advantages of asexuality: which sex limits population growth?Sexual lineageAsexual lineageNote: no figure, but attempted to convey same idea with letters.What would happen if an asexual mutant occurred in a population? Would it increase or decrease?

17 The results of sex: cost of recombinationIf parents have higher than average fitness, what effect of recombination?The results of sex: cost of recombinationA1B1meiosisA2B2parentalrecombinant

18 Experimental test of recombination and adaptationAdaptation may require new combinations of allelesAsexuality does not allow thisExperiment: flour beetles (Tribolium)Have stock populationAllow one population to evolveThe other is restocked from original population, as if asexual.Asexual has 3x reproductive advantageAsexuals start 0.5 of populationSelection: pesticide MalathionWhat proportion are sexual?

20 Do organisms need to adapt? Red queen hypothesisRed Queen to Alice:“Now, here, you see, it takes all the running you can do, to keep in the same place.”--Lewis Carrol,Through the Looking Glass (1872)Concept: constant adaptation needed. Why?

29 Why two sexes? Most species have two sexesSome have multiple sexes (mating types): mating type 1 can mate with anyone except mating type 1, etc.Advantage: higher proportion of population available for matingWhy only two mating types??

36 Evolution of sex ratiosWhich sex should be more common?Why aren’t there many females per male?imagine monogamous speciesfemales become more commonwhich would be better to have has offspring: male or female?

37 Sex bias? If females could choose:What if polygamous species?some males have many matesmost males do not mateIf in excellent condition, should a female produce male or female offspring?If in poor condition, should a female produce male or female offspring?

41 Readings and questions1. In the beetle evolution experiment (figure 8.18) Dunbrack et al did not actually asexual beetles, as there aren't any. Instead they used two different lines of beetles that differed in color, treating one line as if it were asexual by replacing individuals with individuals from a stock population. The researcher's simulated asexual population was not allowed to evolve at all in response to competition and the presence of the insecticide. Is this realistic?2. In general, would you expect asexual lineages to persist longer with small population sizes or large population sizes? Why?3. If the offspring of sexual and asexual individuals have equivalent fitness, why would asexuals take over a population?4. Explain how Muller's ratchet affects sexual populations differently from asexual populations.Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease?In most species, the sex ratio is 50 / 50 male: female. Why wouldn’t evolution favor a higher proportion of females, since one male could mate with many females? Discuss using the idea of frequency dependent selection.